The only guess I can make right off the top of my head is this. The h1 tag by definition is the largest most important element on the page. Making it 90% makes it appear to be less, even if it isn’t.
We would need a link to a page when it’s broken. Then link above is after you set it back to 100%. Also might help if you gave us what stack(s) your using and how your changing the size.
I took a quick look at your site. The syntax error I’m seeing is related to a text stack that has a <span> in it to set the font size to 110%. It then has an unordered list within the span. Don’t know how you’re setting the 110% but you’re not suppose to have a UL within a span.
You can change it just not with a <span> element. From an HTML prospective you could use a <div> or a <section> to wrap your list to apply the class, you can even put the class on the <ul> tag.
The span element is an inline element, intended to be used within a paragraph or header, not to contain block level elements. You could use a span for a list but would need to apply it to each bit of text <li> within the list <ul>.